This application corresponds to Japanese Patent Application No. 10-325029, filed on Nov. 16, 1998, which is hereby incorporated by reference in its entirety.
1. Field of the Invention
This invention relates to a voltage controlled oscillator and to an electronic apparatus using same. More particularly, the invention relates to a voltage controlled oscillator employed in an Electrical Toll Collection (ETC) system used for automatic toll collection, and to an electronic apparatus using same.
2. Description of the Related Art
Radio communication systems using high operating frequencies (approximately 6 GHz), such as current ETC radio communication systems, require a voltage controlled oscillator which serves as a source of a local signal having a frequency close to the operating frequency of the electromagnetic waves used by the system.
A circuit diagram of a conventional voltage controlled oscillator is shown in FIG. 4. FIG. 4 omits circuit elements which are not relevant to a basic discussion of the operation of the oscillator, such as a bias circuit, a choke circuit, and the like.
In FIG. 4, a voltage controlled oscillator 1 comprises of a transistor Q1 for providing oscillation, a resonator R1, a varactor diode (variable capacitance diode) VD, and capacitors C1, C2, C3 and C4. The resonator R1 is a microstrip line which has a xc2xc or less wavelength and has one end grounded.
The base of the transistor Q1 is connected to the resonator R1 via the capacitor C3. Moreover, a connection point between the capacitor C3 and the resonator R1 is connected to the cathode of the varactor diode VD via the capacitor C4, and connected to a control voltage terminal 3. The anode of the varactor diode VD is grounded. In a high frequency band in which a device including the voltage controlled oscillator is used, the collector of the transistor Q1 is grounded via the capacitor C1. The capacitor C1 has low impedance at the oscillation frequency. The emitter of the transistor Q1 is connected to an output terminal 2 via the capacitor C2.
In this voltage controlled oscillator 1, the voltage applied to the control voltage terminal 3 is changed. Based on this change, the voltage applied between the anode and cathode of the varactor diode VD changes. Due to this voltage change, the capacitance between the anode and cathode of the varactor diode VD also changes. A resonance system is constituted by the resonator R1 and the varactor diode VD connected in parallel with the resonator R1 via the capacitor C4. The frequency of the entire resonance system is changed by changing the capacitance between the anode and cathode of the varactor diode VD. As a result, the oscillation frequency of the voltage controlled oscillator 1 can be changed.
The circuit diagram of another conventional oscillator is shown in FIG. 5. FIG. 5 uses the same symbols as FIG. 4 to denote the same or equivalent components, and description of these common components is omitted. Like FIG. 4, FIG. 5 also omits components which are not relevant to a basic discussion of the operation of the oscillator, such as a bias circuit, a choke circuit, and the like.
In the oscillator 5 shown in FIG. 5, the base of the transistor Q1, which is provided for oscillation, is connected to a distributed-constant line 6. The distributed-constant line 6 is an approximately xc2xc wavelength line at the oscillation frequency. One end of the distributed-constant line 6 is open. A collector of the transistor Q1 is grounded via the capacitor C5. The self-resonance frequency of the capacitor C5 is lower than the oscillation frequency of the oscillator 5. The capacitor C5 operates as an inductance element at the oscillation frequency. Generally, a capacitor has a capacitance component, which operates as a capacitance element when the frequency is lower than the self-resonance frequency, and the capacitor has an inductance component, which operates as an inductance element when the frequency is higher than the self-resonance frequency.
In this oscillator 5, since the base of the transistor Q1 is grounded by the distributed-constant line 6 having an open end at the oscillation frequency, the transistor Q1 operates with the base grounded. The capacitor C5 functions as an inductance element. A Colpitts-type oscillation circuit is formed by the capacitance between the emitter and the collector and the capacitance formed between the emitter and the base. The capacitance between the emitter and the collector is not shown, since it is very small and is substituted by the stray capacitance of the emitter side circuit. The capacitance between the emitter and the base is not shown since it is also very small and is substituted by the internal capacitance of the transistor Q1.
The voltage controlled oscillator 1 of FIG. 4 satisfactorily operates at a relatively low frequency. However, as the oscillation frequency becomes higher, it becomes necessary to reduce the inductance value of the resonator R1 and the capacitance value of each capacitor. Variations in operating characteristics become large when the inductance and capacitive values become small. Stray capacitance also exerts a greater influence. Therefore, it has been difficult to obtain stable oscillation, particularly in the microwave band (3 GHz or greater).
On the other hand, stable oscillation of the oscillator 5 in FIG. 5 can be obtained at a relatively high frequency. However, in this range, the oscillator 5 does not operate as a voltage controlled oscillator.
It is an object of the present invention to solve the above-described problems. The present invention provides a voltage controlled oscillator which oscillates in a stable manner at a high frequency and which obtains a large variable range of frequency. The present invention also provides an electronic apparatus using this oscillator.
In order to achieve the above objects, a voltage controlled oscillator is provided comprising a transistor having a base, collector and emitter, for providing oscillation. A distributed-constant line is provided which is connected to the base of the transistor, the distributed-constant line having one open end and having substantially xc2xc wavelength at an oscillation frequency of the oscillator. An inductance element is provided which is connected to the collector of the transistor, and a varactor diode is provided which is connected to the inductance element.
The inductance element can comprise a capacitor having a self-resonance frequency which is lower than the oscillation frequency of the oscillator.
Accordingly to another aspect of the invention, another distributed-constant line is provided having one open end and another end connected to ground, and having substantially xc2xc wavelength at an oscillation frequency of the oscillator. The other distributed-constant line is arranged in parallel with and adjacent to the distributed-constant line which is connected to the base of the transistor.
Further, an electronic apparatus is provided which employs the above-described voltage controlled oscillator.
According to the above configuration, the voltage controlled oscillator oscillates in a stable manner at a high frequency. Also, a large variable frequency range can be obtained. The electronic apparatus of the present invention further offers a reduction in apparatus size and cost.